2010-present Scientist, Japan Atomic Energy Agency(JAEA) 2008-2010 Special Researcher of the Japan Society for the Promotion of Science (JSPS), Japan 2007-2010 Ph. D. student, University of Tokyo, Japan 2007-2008 Junior Research Associate, RIKEN, Japan

Abstract: We discuss the tunneling spectroscopy at a surface in multiband systems such as Fe-based superconductors with the use of the quasiclassical approach. We extend the single-band method by M. Matsumoto and H. Shiba [J. Phys. Soc. Jpn. 64, 1703 (1995)] into n-band systems (n > 2). We show that the appearance condition of the zero-bias conductance peak does not depend on details of the pair-potential anisotropy, but it depends on details of the normal-state properties in the case of fully gapped superconductors. The surface density of states in a two-band superconductor is presented as a simplest application. The quasiclassical approach enables us to calculate readily the surface-angular dependence of the tunneling spectroscopy.

Abstract: Graphene exhibits zero-gap massless-Dirac fermion and zero density of states at E = 0. These particles form localized states called edge states on finite width strip with zigzag edges at E = 0. Naively thinking, one may expect that current is also concentrated at the edge, but Zarbo and Nikolic numerically obtained a result that the current density shows maximum at the center of the strip. We derive a rigorous relation for the current density, and clarify the reason why the current density of edge state has a maximum at the center.

Abstract: We discuss the nuclear magnetic relaxation rate and the superfluid density with the use of the effective five-band model by Kuroki et al (2008 Phys. Rev. Lett. 101 087004) in Fe-based superconductors. We show that a fully gapped anisotropic Â±s-wave superconductivity consistently explains experimental observations. In our phenomenological model, the gaps are assumed to be anisotropic on the electron-like Î² Fermi surfaces around the M point, where the maximum of the anisotropic gap is about four times larger than the minimum.

Abstract: By measuring the angular-oscillations behavior of the heat capacity with respect to the applied field direction, one can detect the details of the gap structure. We introduce the Kramer-Pesch approximation as a new method to analyze the field-angle-dependent experiments, which improves the previous Doppler-shift technique. We show that the Fermi-surface anisotropy is an indispensable factor for identifying the superconducting gap symmetry.

Abstract: To determine the superconducting gap function of YNi2B2C, we calculate the local density of states around a single vortex core with the use of Eilenberger theory and the band structure calculated by local density approximation, assuming various gap structures with point nodes at different positions. We also calculate the angular-dependent heat capacity in the vortex state on the basis of the Doppler-shift method. Comparing our results with the scanning tunneling microscopy and spectroscopy experiment, the angular-dependent heat capacity and thermal conductivity, we propose the gap structure of YNi2B2C, which has the point nodes and gap minima along <110>. Our gap structure is consistent with all results of angular-resolved experiments.

Abstract: On the basis of the quasiclassical theory of superconductivity, we obtain a formula for the local density of states (LDOS) around a vortex core of superconductors with anisotropic pair-potential and Fermi surface in arbitrary directions of magnetic fields. Earlier results on the LDOS of d-wave superconductors and NbSe2 are naturally interpreted within our theory geometrically; the region with high intensity of the LDOS observed in numerical calculations turns out to the enveloping curve of the trajectory of Andreev bound states. We discuss experimental results on YNi2B2C within the quasiclassical theory of superconductivity.

Abstract: We analytically study electronic bound states around a single vortex core in a noncentrosymmetric superconductor such as CePt3Si without mirror symmetry about the ab plane. Considering a mixed spin-singlet-triplet Cooper pairing model, we obtain a formula about the local density of states (LDOS) around a vortex core in any direction of the magnetic field. The LDOS under a magnetic field perpendicular to the c axis is quite different from that of typical s- or d-wave superconductors. From the ellipticity of the spatial pattern of the LDOS around a vortex core, one can experimentally estimate the pairing symmetry of CePt3Si, such as the position of the gap nodes and the ratio of the singlet component to the triplet component in the order parameter.